Deltaic Facies of The Nisqually EarthCache
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Deltaic Facies of The Nisqually
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The delta of the Nisqually River is located within the U.S. Department of Fish and Wildlife’s Nisqually National Wildlife Refuge. The refuge is northeast of Olympia and easily accessible from I-5 at exit 114.
To accomplish the objectives of this Earth Cache, you will need access to the Refuge and its board walk trail. This Earth Cache provides an opportunity to learn about geological characteristics of deltas.
This is a fee access location. The site is open during daylight hours; except certain holidays. Be prepared to walk about four (4) miles to complete this Earth Cache.
The headwaters of the Nisqually River originate at Mount Rainier’s Nisqually Glacier at an elevation of 4,809 feet above sea level. As the river moves through west central Washington, it is supplied with additional flow from the Little Nisqually and Mashel Rivers. Groundwater also discharges into the river as it flows to its mouth in Puget Sound. All together, the Nisqually River basin includes drainage from an area of 517 square miles. Its average flow is 1460 cubic feet of water per second or 655,250 gallons/minute. . Its average annual maximum flow is 39,500 cubic feet per second or 152,367,600 gallons per minute and its minimum flow is 460 cubic feet per second which is equal to 206,448 gallons per minute.
As the river flows through its basin it transports millions of pounds of sediment towards its mouth in Puget Sound. This transported sediment consists of three components: a dissolved portion, a suspended portion and bed load. The dissolved load consists of substances derived from chemical weather as a consequence of solubility. Approximately ten percent of the total transported material in rivers and streams consists of dissolved load. The suspended load consists of fine grained particles that once eroded, settle very slowly and are transported large distances. About sixty six percent of the total transported material in rivers and streams consists of suspended load. The bed load moves by rolling and sliding along the bottom. The bed load consists of larger grain sized particles such as pebbles, cobbles, boulders etc. As a rule, the bed load becomes courser and larger near the head waters and the stream bed becomes more fine grained as the stream progresses towards its mouth. About a quarter of the total transported material of rivers and streams consists of the bed load portion.
The Washington Department of Ecology estimated this suspended sediment load to be 2050 tons/day in 2007. Therefore, the Nisqually River discharges about 1,700 cubic yards of suspended sediment a day at its mouth.
The Nisqually Delta is the result of 2050 tons per day of sediment (or 1708 cubic yards) of sediment being continuously deposited as the river enters Puget Sound. This deposition occurs because as the river water enters the sound, its velocity is reduced. When this happens abruptly, deltas are formed.
While some of the suspended sediment is transported away from the delta by currents in Puget Sound, there is equilibrium between the input of suspended sediment, expansion of the delta and erosion.
Geologists call the period of delta growth or expansion through deposition the Constructional Phase of delta development. However when tidal current and wave action remove more sediment than the river can supply, the delta is in what is called a Destructional Phase of delta development. The Destructional Phase of a River delta can be inadvertently triggered by changes in the basin which alter the flow characteristics such that velocities decrease and the river loses its ability to transport sufficient quantities of sediment to maintain balance in the delta between sediment erosion and deposition. One way this can happen is through the creation of dams that cause river velocity to drop.
River deltas that form in marine environments produce extremely important habitats and resource rich environments. Geologists have identified four unique environments called facies associated with deltaic systems. Sedimentary facies consist of sediments that are recognizably different and distinct from adjacent sediment deposited in a different depositional environment. These include:
a) Upper Delta Plain which extends from the flood plain of the river to the most landward limit of tidal coverage of the land surface (which is called tidal inundation). This is a freshwater dominated environment with braided channels and meandering channel deposits of sand. We would expect to see swamps, marshes and even (in larger deltas) freshwater lakes in the Upper Delta Plain. The sediment present would likely consist of a mixture of sand, silt and clay along with organic material. It would be a fertile soil.
b) Lower Delta Plain includes the areas of the delta which are typically submerged during most tides. The width of the Lower Delta Plain is determined by tidal range; where tidal range is large the width of the Lower Delta Plain is increased. Because this area is intertidal, it can be readily observed during times of low tide. In this environment we would expect to see numerous drainage features with a characteristic branching (or dendritic) pattern. This is an environment of mixing fresh and salt water; it is a shallow estuarine habitat. The sediments present would be sand of a more uniform composition.
c) Subaqueous Delta Plain lies seaward of the low tide level. It is always covered by water. The water is likely brackish this environment is also characterized as estuarine. This area is principally comprised of sand which grades finer towards seaward.
d) Prodelta is the seafloor seaward of the Subaqueous Delta Plain. It is always submerged and tends to be more marine than estuarine in terms of the type of water present. The sediments in this area tend to be finer grained than those present in the Subaqueous Delta Plain with greater amounts of silt and clay present.
Reference
Boggs, S. (1995) “Principles of Sedimentology and Stratigraphy (2nd) “ Prentice Hall, N.J.
To claim this Earth Cache there are several things you will have to do:
Walk the trails to the Waypoints specified. Make the requested observations and decide which of the deltaic facies is present at the location. Send your answers to the Earth Cache owner.
Stay on the trails and boardwalk. Enjoy all the sights and sounds! You might even see bald eagles in the vicinity.
1) The First Location for determining the deltaic facies is located on the Twin Barns Loop Trail at
N 47⁰ 04’ 28.1” W 122⁰ 42’ 46.6”
2) The Second Location for determining the deltaic facies is located on the Nisqually Estuary Trail at
N 47⁰ 04’ 52.7” W122⁰ 43’ 2.4”
3) The Third Location for determining the deltaic facies is located on the Nisqually Estuary Boardwalk Trail at
N 47⁰ 05’ 30.0” W 122⁰ 43’ 33.17”
4) The Fourth Location is inaccessible…but is observable from the Third Location: its due north 1.5 miles of the Third Way Point. (Hint: It’s not Prodelta! And the trail map might be useful in determining what facies is likely at this location)
Additional Hints
(No hints available.)